Construction methods · Shallow foundations, cofferdams and dewatering

Double-wall and cellular cofferdams

Two rings of sheet piles with sand or gravel fill between them, or a chain of filled circular cells, stand as gravity structures against 10-20 m of water without internal bracing. They are the cofferdam of choice for large pier footprints in deep, fast water.

How bridges get built · Shallow foundations, cofferdams and dewatering · Double-wall and cellular cofferdams

Also called: cellular sheet pile cofferdam, double-wall steel cofferdam, circular cell cofferdam

How it is done

Drive two concentric lines of sheet piles (or flat-web piles for circular cells) from a template; tie the walls with rods or rely on the cell geometry.
Fill the annulus or cells with sand by clamshell or hydraulic placement.
Excavate the interior, place a tremie seal if needed, dewater and construct the foundation with an open floor free of struts.
Cells are sometimes left in place as part of the pier protection.

Where it fits

Piers in 10-20 m of water, large footprints for pylons, sites where internal struts would obstruct the work.

Where it does not

Very soft foundation beds where the filled wall would slide; rock heads that prevent embedment.

Plant, pace and money

PlantSheet piles, templates, vibro and impact hammers, filling plant, barges.

ProductivityWeeks to months per cofferdam; a large cellular cofferdam can take half a year.

CostUSD 2-10 million per major pier.

Risks and controls

What goes wrong

Cell rupture during filling, sliding under flood, interlock tension failure.

Quality assurance

Interlock tension design checked, fill density monitored, deformation surveyed daily during dewatering.

Examples

Mississippi and Ohio river piers; large estuary bridges in Japan.